This application is the U.S. national phase of International Application Ser. No. PCT/GB99/04306, filed Dec. 17, 1999, which designated the U.S., the entire content of which is hereby incorporated by reference.
The present invention relates to semiconductor avalanche photo-diodes, and in particular to single photon avalanche diodes (SPADs).
Semiconductor avalanche photo-diodes are currently used in optical communications and for light detection in general. An avalanche photo-diode (APD) comprises a p-n structure which is operated under high reverse bias.
In some structures, light is absorbed within the avalanche region, but for long wavelength applications at eye safe wavelengths a separate absorption and multiplication (SAM) detector is often preferred. A photon detection region is sensitive to incident light to inject carriers into the conduction band of a depleted avalanche region of the photo-diode. The injected carriers traversing the avalanche region gain sufficient energy to enable further carriers to be excited across the energy gap between the valence and conduction bands by impact ionisation. This generates new carriers which themselves may generate further carriers by the same process. This process is known as avalanche multiplication of carriers and current gains in excess of 100 are readily obtainable.
A SPAD device is sensitive enough to detect a single incident photon of light. A single photon incident on the device will excite an electron from the valence band into the conduction band and the avalanche multiplication process will then generate a measurable pulse of current. However, SPAD detectors can generate pulses of current or dark counts which are not generated by an incident photon of light but are instead generated by the thermal release of a carrier from a trap in the semiconductor lattice. This necessitates the cooling of the SPAD device.
Furthermore, the avalanche multiplication process can be very noisy and this can compromise the performance of the SPAD detector by increasing jitter and reducing time resolution.
U.S. Pat. No. 3,453,436 discloses an avalanche photo-diode detector arrangement in which a photo-diode is biased to below its breakdown voltage and in which an oscillating voltage is applied to the photo-diode so that the photo-diode periodically exceeds its breakdown voltage. The frequency of the oscillating voltage is 200 kHz, which frequency is chosen so that the generation of microplasmas within the diode structure is supressed by ensuring that the voltage applied to the diode exceeds the threshold voltage for the generation of microplasmas for a period less than the period of time for activation of a microplasma.
The present invention aims to overcome at least some of the above mentioned problems by providing a avalanche photo-diode arrangement which can discriminate against dark counts and so relax the requirement for cooling of the photo-diode and/or which can reduce the noise of the avalanche multiplication process.
According to a first aspect of the present invention there is provided an avalanche photo-diode arrangement comprising;
at least two avalanche photo-diodes, which are each
reverse biased to just below their breakdown voltage, and
at least one oscillating voltage source arranged such that
an oscillating voltage is applied to each photodiode so that
each photo-diode exceeds its breakdown voltage periodically,
wherein the or each oscillating voltage has a period greater than twice the avalanche zone transit time of the photo-diode to which it is applied.
By arranging a plurality of photo diodes in this way each diode will generate a current pulse in response to an incident light signal If the latter has a duration at least as long as one period of the voltage oscillation. For dark count rates which are low compared to the frequency of the oscillating voltage, a dark count event will generate a current pulse in only one of the detectors and in this way dark counts can be discriminated. Also, by arranging the oscillating voltage source to have a period greater than twice the avalanche zone transit time of the photo diodes, low noise operation of the photo-diodes can be achieved.
By choosing the period of oscillation P of the oscillating voltage to be in a preferred range of 4T to 32T, where T is the avalanche zone transit time of the associated photo-diode, it has been found that current pulses are generated with very low noise. For periods less than 4T, the probability of multiple pulses from dark counts becomes appreciable as the chance of carriers persisting in the diode from one cycle to the next starts to grow. For P greater than 32T, simulations show that the avalanche noise levels start to rise. For example, at the optimum period of 4T, the excess avalanche noise factor is close to 2.6 and fairly insensitive to multiplication level (or voltage swing). For a period of 64T this has risen to, ≈18 and for longer periods the excess noise factors approach values available for DC biased diodes. For some applications it may be necessary to tolerate higher noise levels in order to achieve higher photocurrent. The period chosen for the oscillating voltage will in practice depend on the particular application. In general the shorter P gives the lowest excess avalanche noise and the longer P gives the higher photo current for a given amplitude of voltage oscillation.
The photo-diodes used in the arrangement may be made of group IV semiconductor materials, for example of a Silicon Germanium photon detection region and a Silicon avalanche region. Alternatively, the photo diodes used in the arrangement may be made of III-V semiconductor materials, for example of an Indium Gallium Arsenide photon detection region and a Indium Phosphide avalanche region.
Preferably, the photo diodes exceed their breakdown voltage out of phase with each other in order to generate a series of pulses In response to an incident light signal which can be discriminated from a single pulse which would be generated by a dark count event.
In an especially preferred embodiment two photo-diodes are used and are arranged back-to-back and in series with an oscillating voltage source. When a light signal is incident on the arrangement it will excite at least one electron from the valence band into the conduction band in both of the photo-iodes, however, avalanche multiplication will only occur in the photo-diode which is reverse biased above a threshold voltage.
When a light signal having a duration at least as long as the period of the oscillating voltage is incident on this preferred arrangement using two photo-iodes, it will generate a series of at least two pulses of current which are separated by half the period of the oscillating voltage. When the oscillating voltage is in its positive half cycle, the reverse bias voltage of a first one of the photo-diodes rises above its breakdown voltage and so can undergo avalanche multiplication to generate a first current pulse. At the same time the reverse bias voltage of a second of the photo-diodes falls further below its breakdown voltage and so cannot undergo avalanche multiplication. Then, when the oscillating voltage is in its negative half cycle, the reverse bias voltage of the second photo-diode rises above its breakdown voltage and will undergo avalanche multiplication to generate a second current pulse, delayed relative to the first by a time equal to half of the period of the oscillating voltage. At the same time the reverse bias voltage of the first photo diode falls further below its breakdown voltage and so cannot undergo avalanche multiplication.
If an incident light signal is incident on a photo-diode which is in its negative half cycle (ie. has a reverse bias voltage below the threshold), then the probability of detection in this diode is low. Consequently, in the preferred arrangement using two photo-diodes only 50% of the input signal is multiplied up. For most applications the extremely low noise levels achieved and the very high multiplication levels which become possible by using the arrangement according to the present invention, more than compensate for the loss of signal.
The total charge generated per photon will have extremely low noise levels making the arrangement according to the present invention ideal for use in fibre-optic communications to detect signals with bit rates approaching the oscillation frequency of the oscillating voltage. In fibre-optic communications the injection current generated by an incident light signal will in general be high relative to the injected current for a dark count event and so discrimination of dark count events is not such a big problem.
Discrimination against dark counts is, however, of very high interest for very low levels of incident light signals. In this case the signal duration must extend over at least one period of oscillation, so that both diodes generate at least one output pulse. The minimum number of photons which can be detected is therefore xe2x89xa72.
Thus, the arrangement according to the present invention using two photo-diodes generates a minimum of two current pulses in response to an incident light signal which has a duration at least as long as a full cycle of voltage oscillation. Thus, an incident signal can be distinguished from a dark count event which will occur in only one of the photo-diodes and so will generate only one current pulse.
In this preferred embodiment the two photo-diodes are preferably substantially 180xc2x0 C. degrees out of phase so that one of the diodes is xe2x80x9copenxe2x80x9d for detection at all times. Having the diodes 180xc2x0 C. out of phase is also useful for minimising rf in the output of the photo-diode arrangement. Otherwise, the quantum efficiency of the arrangement would be reduced and compensating circuitry might be necessary to minimise the rf in the output of the arrangement. If the output rf from the oscillator circultry is small compared to the photocurrent generated by the photo-detectors, then this is less of a problem. Further, in applications where a reduction in detection probability can be tolerated the diodes need not be exactly 180xc2x0 C. out of phase.
It is preferred that the oscillator is a balanced oscillator and that the two photo-iodes are identical, so that the oscillator does not feed significant radio frequency (rf) current to the output of the arrangement. Alternatively, if the photo-diodes are not sufficiently close in rf impedance, then some tuning circuitry could be included for matching impedance of the photo-diodes and to minimise rf output from the arrangement.
In the preferred embodiment with two photo-diodes a light signal incident on the arrangement is preferably split into two, preferably equal, parts which parts are coupled to respective photo-diodes in the arrangement. This can be done by beam splitters or by using waveguide techniques.
A simplified arrangement is achieved when the two photo-diodes have the same structure.
According to a second aspect of the present invention there is provided a method of detecting incident light signals using an avalanche photo-diode arrangement comprising the steps of:
reverse biasing at least two photo-diodes to just below
their breakdown voltage,
applying an oscillating voltage to each of the photo-diodes
such that each of the photo-diodes exceed their breakdown voltage periodically,
wherein the or each oscillating voltage has a period which is greater than twice the avalanche zone transit time of the photo-diode to which it is applied.
Preferably, the photodiodes exceed their breakdown voltage out of phase with each other.
Preferably, the oscillating voltage has a period of between 4 and 32 times the avalanche zone transit time.
It is also preferred that the light incident on the arrangement has a duration of at least one period of the oscillating voltage.
In a preferred embodiment the arrangement comprises two photo-diodes which are arranged back-to-back and in series with one oscillating voltage source.
This latter preferred embodiments preferably comprises the additional steps of:
splitting an incident light signal into two, preferably equal parts, and
coupling the two parts to a respective one of the photo-diodes so that the parts are incident on the photo-iodes in phase.